The catalytic preparation of hydrocarbons from synthesis gas, i.e. a mixture of carbon monoxide and hydrogen, is well known in the art and is commonly referred to as Fischer-Tropsch synthesis.
Catalysts suitable for use in a Fischer-Tropsch synthesis process typically contain a catalytically active metal of Group VIII of the Periodic Table of the Elements (Handbook of Chemistry and Physics, 68th edition, CRC Press, 1987–1988) supported on a refractory oxide, such as alumina, titania, zirconia, silica or mixtures of such oxides. In particular, iron, nickel, cobalt and ruthenium are well known catalytically active metals for such catalysts. Reference may be made to EP-A-398420, EP-A-178008, EP-A-167215, EP-A-168894, EP-A-363537, EP-A-498976 and EP-A-71770.
There is a continuous interest in finding catalysts for use in the Fischer-Tropsch synthesis which provide an improved activity and an improved selectivity in the conversion of carbon monoxide into valuable hydrocarbons, in particular hydrocarbons containing 5 or more carbon atoms (“C5+ hydrocarbons” hereinafter), and which minimise the formation of methane, which is a hydrocarbon carbon frequently considered as being of lower value.
U.S. Pat. No. 5,545,674 discuses the use of shell catalysts in the Fischer-Tropsch synthesis. These catalysts have the catalytically active metal positioned exclusively in a relatively thin outer layer of the catalyst particles. Compared with catalysts which have the catalytically active metal evenly dispersed throughout the catalyst particles, the shell catalysts have a short diffusion length and they are low in diffusion limitation, and therefore they show a relatively high selectivity with respect to the formation of C5+ hydrocarbons, and they suppress the formation of methane.
Apart from Fischer-Tropsch synthesis processes, shell catalyst may be used in other chemical conversion processes, in particular where diffusion limitation plays a role.
For a high reaction rate or reactor productivity it is desirable to have a large quantity of the catalytically active metal in the outer layer of the shell catalyst particles. According to U.S. Pat. No. 5,545,674 and references cited therein, this can be achieved by repeated impregnation of a solution comprising the catalytically active metal into the carrier particles, using an immersion or a spraying method, with intermediate drying steps and calcination steps. Not only is this multi-step process cumbersome and time consuming, but by applying the (repetitive) impregnation method, some of the catalytically active metal may penetrate into layers of the catalyst particles which are deeper then desirable, in which case the shell catalyst adopts more of the characteristics of a catalyst which has the catalytically active metal dispersed evenly throughout the catalyst particles.